On 1st December 2015 Transport for London (TfL) unveiled its new design bible, the Design Idiom. Though the name may sound grandiose, the goal is simple: create a document that captures the design aesthetic of the Underground, so that good design can help drive decision-making at London Underground.
The document itself can be found here (PDF), and I whole-heartedly recommend reading through every part it. It’s an excellent example of a set of design guidelines that isn’t touchy-feely and alight with cockamamie buzzwords. Instead, it is the product of proper research that provides concrete examples of how to implement each high-level concept. In essence, it explains why the guidelines matter.
The article is great, too.